1. Field of the Invention
This invention relates to a process for the fabrication of a metal-made carrier body for an exhaust gas cleaning catalyst which is generally interposed as an exhaust gas cleaning means at an intermediary point of an exhaust system of an automotive vehicle. More specifically, this invention is concerned with a process for the fabrication of a metal-made carrier body whose principal components--such as one or more sheet-like metal bands made of thin metal sheets and one or more corrugated metal bands and optionally, a metal-made casing--are firmly brazed together, so that the metal-made carrier body remains intact over a long period of time and exhibits excellent properties in vibration proofness and the like.
2. Description of the Related Art
As carrier bodies for an exhaust gas cleaning catalyst of the above sort, there have conventionally been known metal-made carrier bodies which can withstand heat (high temperatures), impacts and vibrations in use and can also carry a catalyst in a large amount per unit volume. In order to fabricate such metal-made carrier bodies, at least one sheet-like metal band and at least one corrugated metal band, each of said bands being made of a thin metal sheet, are superposed one over the other so as to establish contacts therebetween. Thereafter, they are rolled together into a spiral form or are laminated in layers, thereby forming a multi-layered composite body defining a number of network-patterned gas flow passages along the central axis thereof. If necessary, the multilayered composite body is enclosed in a cylindrical metal-made casing which are open at both ends thereof. The contacts between the sheet-like metal band and corrugated metal band and between the composite body and the cylindrical metal casing are then bonded.
The above bonding may be conducted, for example, by electron beam welding, laser welding or arc welding as disclosed in Japanese Patent Application Laid-Open Nos. 13462/1979, 1585/1982 and 4373/1981 by way of example, by applying a paste of a binder and a brazing filler metal to the surface of at least one of the sheet-like metal band and corrugated metal band, by applying a binder to the surface of at least one of the sheet-like metal band and corrugated metal band and then applying powder of a brazing filler metal further, or by applying a sheet-like solder (brazing material).
The above-described bonding methods, which have conventionally been employed upon fabrication of metal-made carrier bodies, are however accompanied by the following problems.
The former methods such as electron beam welding and laser welding require a special and expensive apparatus, and the productivity is unavoidably reduced due to the need for cumbersome and complicated work involving spot welding at narrow superposed contact portions or inner contact portions.
Brazing requires the use of a costly brazing material. In addition, complex and time-consuming work is indispensable upon coating and drying the brazing material. If planar band and corrugated band, at least one of said bands being coated with a brazing material, are superposed one over the other and are then rolled together into spiral form to form a multi-layered composite body, the composite body is then enclosed in a cylindrical metal casing, and contacts between the planar band and corrugated band are then brazed, the dimensions of the components vary compared to their dimensions before the brazing so that the rolling may become looser or a gap may be formed at the contacts. This can be attributed to the fact that the coating layer of the brazing material present at the contacts between the planar band and corrugated band is fused away. Since the composite body is rolled in many layers, the loose and extra length becomes substantial so that problems would arise in vibration proofness. A metal-made carrier body of the above sort has an extremely large potential problem that when actually incorporated in the exhaust system of an automotive vehicle, gaps would be formed due to the high-temperature environment of the exhaust gas as the time goes on, to say nothing of the above heat treatment. This problem also arises even when either one or both of the planar and corrugated bands is plated with the brazing metal instead of coating it with the brazing material. Even when constituent materials with a coating layer of a brazing material formed in advance on a surface thereof are used and are thereafter brazed subsequent to their assembly into a metal-made carrier body, the bonding of the constituent materials is insufficient and the thus-fabricated metal-made carrier body does not have excellent vibration proofness.